1. Field of the Invention
The present invention relates generally to mobile marine structures, and more particularly to a mobile marine structure having a cantilever assembly extendable over the side of the structure with hydraulic load equalizer apparatus for distributing stresses between the cantilever assembly and the structure.
2. Description of the Prior Art
In the drilling of oil and gas wells in a marine environment, a number of mobile structures or rigs have been developed. A particularly useful rig for drilling in relatively shallow water is the jack-up rig. A jack-up includes a floatable hull with at least three vertically extendable legs. The rig is floated to the drill site and then the legs are extended to contact the sea bottom and raise the hull out of the water, thus providing a stable platform from which drilling operations may be conducted.
An improvement in the jack-up rig field is what is known as the cantilever rig. Cantilever rigs are generally jack-up rigs that have the well drilling equipment mounted on a cantilever assembly which is slidingly mounted on the deck of the structure, but submersibles, bottle stabilized, posted and other types may also use the cantilever. The cantilever rig is spotted adjacent the drill site and the drilling equipment may be moved onto and off of the actual drill site without moving the rig.
A problem with cantilever rigs has to do with the tremendous stress concentrations between the cantilever assembly and the outward edge of the rig. The loading on the cantilever end of the cantilever assembly, including the weight of the derrick, drawworks, related pipe handling equipment, and load on the hook, can amount to thousands of tons. That weight is counter balanced by a hold-down structure holding down the opposite end of the cantilever assembly. While, in common experience the massive horizontally extending steel beams of the cantilever assembly are thought to be rigid, with such tremendous loadings, the beams actually flex or bend. Thus, the cantilever beams are not supported on the deck of the rig for their entire inboard length. Rather, the area of contact between the loaded cantilever beams and the deck is quite small and approximates a fulcrum or point support. Thus, the unit pressure on the loaded area of the beams and deck structure is extremely high. These extreme stress concentrations can result in gouging or galling of the metal surfaces during sliding, and in some cases to mechanical failure of the beams or deck.
An attempted solution of the stress concentration problem is provided in U.S. Pat. No. 3,477,235. In the structure of that patent, there is provided at the outboard edge of the deck a rocker bearing assembly. The rocker bearing assembly includes a rocking bearing plate pivotally mounted to the structure. The rocker bearing plate may thus pivot to conform to the bottom of the cantilever beam, thus spreading the stress over the entire length of the rocker bearing plate. However, the entire load on the rocker bearing plate is transferred to structure through the bearing. Thus, all of the stress is concentrated in the bearing.
In additional but related problem in cantilever rigs is in the fact that the outboard end of the cantilever assembly sags and is not level. Thus, the derrick is not vertical and the hoisting gear is not plumb with the center line of the well. While the structure of U.S. Pat. No. 3,477,235, provides some abatement of stress concentration on the beams, it provides no means for leveling the outboard end of the cantilever assembly.
It is therefore an object of the present invention to provide a uniform distribution of a heavy load over a large area, particularly the load between a cantilever beam and a supporting surface, thereby reducing the unit loading therebetween.
It is a further object of the present invention to provide a uniform distribution of the load within a wide range of angles of declivity of the loaded member.
It is a further object of the present invention to provide an apparatus for raising or jacking a supported load in such a way that the load is uniformly raised and uniformly supported while being raised.
It is a further object of the present invention to provide a load supporting system that can be adjusted manually with tools that are readily available in the field.
It is a yet a further object of the present invention to provide means for determining the magnitude of the load that is being supported.